the Mondofiedo nappe (NW Spain) - E

Large extensional structures developed during emplacement of a
crystalline thrust sheet: the Mondofiedo nappe (NW Spain)
bl
Jose R. Martinez Catal<ina,*, Ricardo Arenas , , Mafia A. Diez Baldaa
"Departamento de Geologta, Universidad de Salamanca,
b
37008
Salamanca, Spain
Departamento de PetroZogta y Geoqutmica, Universidad CompZutense,
28040 Madrid,
Spain
Abstract
The Mondofiedo nappe is a crystalline thrust sheet characterized by large recumbent folds, regional intermediate-pressure metamorphism,
synkinematic intrusion of granitoids during nappe emplacement, and an extensional ductile shear zone developed within the nappe during
thrusting. A large tectonic window permits the study of the footwall unit, revealing another extensional shear zone contemporaneous with
thrusting and a low-pressure metamorphic evolution, in contrast to that of the hanging wall unit. The two main extensional shear zones
produced E- W extension parallel to the direction of orogenic shortening and normal to the orogenic structural trend. Furthermore,
subordinate N -s longitudinal extension was accommodated by normal faults in the footwall, and some of these faults were used as lateral
ramps in late stages of thrusting.
The role of the extensional shear zones and faults described is discussed in the context of an evolving orogenic wedge dominated by plate
convergence but characterized by large-scale rheological heterogeneities within it. Deep-seated viscous flow, triggered by heat accumulation,
seems to account for the horizontal stretching and probable tapering of the orogenic wedge, which was induced by gravitational instabilities
due to partial melting and underplating by buoyant continental crust.
Keywords: Crystalline nappe; Syn-orogenic extension; Orogenic wedge; Iberian Massif
1. Introduction
surface, in order to establish original fault dips or make
estimates of displacement and shortening. Furthermore,
Studies of thrust sheets within the internal zones of
many internal nappes are very large (larger than the largest
orogenic belts are important in assessing crustal conver­
foreland thrust sheet; Hatcher and Williams, 1986) and their
gence, and estimating the long-term rheological behaviour
outcrop is
of continental lithosphere.
structures. Due to these problems, large associated struc­
However,
the emplacement
obscured by plutonic intrusions and late
history of crystalline thrust sheets is usually complex and
tures are typically assumed to pre- or post-date thrusting,
difficult to ascertain, partly because they involve large
instead of investigating the possibilities of being coeval
portions of the crust (Hatcher and Hooper, 1992), and even
with it.
the upper mantle, and include a wide range of rheological
Structures whose kinematics do not fit the expected
behaviours along a single allochthonous unit. Descriptions
compressional dynamic framework, or omit parts of the
of large-scale thrust geometry and associated macrostruc­
normal stratigraphic sequence or metamorphic succession
tures less commonly focus on the internal zones of orogenic
appear often associated to crystalline nappes. In some cases,
belts than in the external zones. Difficulties arise because
they could be structurally induced by the geometry of the
crystalline thrust sheets involve rocks previously folded, so
that the bedding cannot be taken as a horizontal reference
+34-923-294488; fax: +34-923-294514.
E-mail addresses : j rmc@usal .es (J.R. Martfnez Catahin)., arenas@geo.
ucm.es (R. Arenas), [email protected] (M.A. Dfez Ba1da).
1 Tel.: +34-91-3944908; fax: +34-91-5442535.
*
Corresponding author. Tel.:
thrust fault (Dahlstrom, 1970; Coward, 1982; Wibberley,
1997), linked to the development of imbricates, out-of­
sequence thrusts or tear faults (Butler, 1982; Morley, 1988;
Mueller and Tailing, 1997), or be equivalents to Riedel
shears oblique to the basal thrust (Yin and Kelty, 1991). But,
in other cases, their size, in relation to the nappe to which
they are associated, and their imprint in its metamorphic
evolution point to large-scale phenomena, related to the
dynamics of the whole orogenic belt rather than to the
individual allochthonous units.
In particular, extensional structures have been found in a
number of large crystalline thrust sheets in the Scandina vi an
Caledonides (possen, 1992, 2000; Milnes et aI., 1997), the
Enropean Variscides (Bnrg et aI., 1994; Pitra et aI., 1994),
the North American Cordillera (Hodges and Walker, 1992)
and the Himalayas (Bnrg et aI., 1984; Hodges et aI., 1996),
among others orogenic belts. There, much attention has
been paid to the syn- or postorogenic character of the
extensional structures. For instance, Fossen (2000) pointed
out that whereas in the Caledonides, extension is essentially
postorogenic, the Himalayas offer good examples of
synorogenic extension.
The problem with synorogenic extension is how a local
stress field can develop opposite to the regional or distant
convergence-related stress field. Commonly invoked expla­
nations are the upwelling of some hot and buoyant material,
such as the asthenosphere or molten crust and mantle rocks
(Van Den Driesche and Brnn, 199 1-1992; Vanderhaeghe
et aI., 1999), and the creation of a high topographic relief,
which may develop vertical stresses high enough to make
nnstable and collapse a large portion of the monntain belt
(England, 1983; Bnrg et aI., 1984; Bnrchfiel and Royden,
1985; Dewey, 1988; Molnar and Lyon-Caen, 1988). In the
latter case, the relief may simply result from crustal or
lithospheric thickening due to convergence, or be a
consequence of the removal of the mantle lithospheric
root either by detachment or by convective erosion (Platt
and Vissers, 1989; Platt, 1993).
Much of the discussion on synorogenic extension in
mountain belts is linked to that of exhumation of deep­
seated structural units, often showing evidence of high­
pressure metamorphism. The quick exhumation rates
required for the preservation of high-pressure parageneses
is difficult to explain by erosion only, and has been linked to
tectonic denudation in many cases (Davies and Warren,
1988; 101ivet et aI., 1998). To account for the synchronism
often observed between exhumation and convergence, Platt
(1986) discussed the mechanics of orogenic wedges
consisting of viscous material, typical of the internal
zones of orogenic belts. Although inspired in the well­
established mechanics of accretionary wedges with Cou­
lomb behavionr (Davis et aI., 1983; Dahlen et aI., 1984), he
avoided choosing a particular bulk rheology, and carried out
a merely qualitative analysis. From it, he suggested that
transitions from local convergence to local extension could
be driven by changes in wedge geometry, and showed how
extension may occur in parts of the wedge during
continuous convergence.
Platt's conclusions pertaining to tectonic denudation do
not differ essentially from the models relating extension
with gravitational collapse, but provide mechanisms by
which the wedge can be thickened and turned unstable,
namely changes in its internal rheology or in the rate of
convergence, and underplating by continental slices.
Obviously, the generation of molten, buoyant material
represents a drastic rheological change inside the wedge
(which may modify the local stress field), and the addition
of mantle material may be a kind of magmatic
underplating.
The idea that adjustments inside the orogen, including
extensional structures, is related to viscous flow in its deep
parts, is gaining adepts progressively. Although primarily
seen as a post-convergence phenomenon (Sandiford, 1989;
Block and Royden, 1990; Van Den Driesche and Brnn,
199 1-1992; Costa and Rey, 1995), it can overlap with
orogenic shortening if convergence continues after the
middle and/or lower crust have had enough time to develop
a low-viscosity hot layer (Clark and Royden, 2000;
Beaurnont et aI., 2001; Shen et aI., 200 1).
The aim of this article is to demonstrate that large
extensional structures in the Mondoiiedo nappe and its
autochthon formed while Variscan convergence was active
and, mostly, dnring the emplacement of the nappe itself.
Extension was mainly transversal to the structural trend; that
is, it took place in the direction of nappe emplacement­
although in two opposite senses-but subordinate extension
occurred also longitudinally. Extensional and compressive
structures interfered during nappe emplacement, and cross­
cutting structural criteria, together with overprinting
metamorphic relationships, information extracted from
synkinematic granitoids, and published age data, permit a
detailed reconstruction of the evolution of this area of the
Iberian Massif. The deduced tectonothermal history is then
discussed in the context of the Variscan orogenic wedge and
its kinematics, which includes the role of viscous flow
associated with high-grade metamorphic rocks developed
below the nappe.
2. Setting and previous work
The Mondoiiedo nappe is a large crystalline thrnst sheet
occurring in the internal zones of the NW Iberian Massif,
and made up of low to high-grade metasedirnents and
several massifs of synkinematic granitoids (Figs. 1 and 2). It
was first described by Matte (1968) as a large recumbent
anticline, and then by Marcos (1973), who mapped its
frontal thrust and stated its importance as a first-order
allochthonous unit of the Variscan internal zones. The
southern branch of the nappe was mapped by Perez-Estaun
(1978), and further research along the coastal section
showed the existence of a 3 -3.5-km-thick basal ductile
shear zone (Figs. 3 and 4) in its internal parts (Bastida and
Pulgar, 1978). Martinez Catalan (1980) identified the basal
thrnst outcropping in these internal parts and mapped the
thrust and associated shear zone, showing the existence of
two tectonic windows, named Xistral and Monte Carballosa
(pig. 1). The thrnst front can be traced for 200 km and a
CANTABRIAN
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43'
BIOTITIC POSTKINEMATIC
GRANITOIDS
TWO·MICA SYN· TO POSTKINEMATIC
GRANITOIDS
BIOTITIC SYNKINEMATIC
GRANITOIDS
Cl
-
20 km
10
I==--==-_
o
Fig.
1.
Cl
�
EIIJ
Cl
-
SECEDA, GARGANTA AND lUARCA FM.
LOWER DEVONlAN-UPPER ORDOVICIAN
UPPER CABOS·ARMDRICAN QUARTZITE
ARENIG
CABOS SERIES
MIDDE CAMBRIAN·lOWER ORDOVICIAN
VEGADEO LIMESTONE
LOWER CAMBRIAN
TRANSITION BEDS
LOWER CAMBRIAN
UPPER CANDANA QUARTZITE AND
XISTRAL QUARTZITE. LOWER CAMBRIAN
CANCANA SLATES AND LOWER CANCANA
QUARTZITE. LOWER CAMBRIAN
VILALBA SERIES
UPPER PROTEROZOIC
HOLlO DE SAPOR FM.
ORTHOGNEISSes
Geological map of the northern and central parts of the Mondonedo nappe.
LITHOLOGY
2000 m
Rhyolite
1500
Carbonaceous slate
1000
Slate
Slate and greywacke
500
Sandstone
Quartzite
o
MONDONEDO
NAPPE
Carbonate
XISTRAL
TECTONIC
WINDOW
XISTRAL
QUARTZITE
LOWER
CAMBRIAN
QUARTZITE
VILALBA SERIES
Fig.
2.
Stratigraphy of the Mondonedo nappe and its autochthon (Xistral
tectonic "Window). The key to the stratigraphic divisions (left hand colunms)
is given
in Fig. 1.
width of 65 km is measurable for the thrust sheet between its
frontal thrust and the western limit of the tectonic windows.
Displacement cannot be measured because of the absence of
correlatable cut-offs between the hanging wall and the
footwall. However, correlation between two Lower Cam­
brian quartzite formations (Upper Caudana and Xistral
quartzites; Fig. 2), permits a minimum estimate of 45 km
(see cross-section A-A' in Fig. 4).
The nappe consists of 3000 m of Upper Proterozoic
slates and greywackes, the Vilalba Series, and another
3000 m of Lower to Middle Paleozoic clastics and
carbonates of shallow-water platform facies, locally reach­
ing the Lower Devonian (Fig. 2). Slates are the most
common lithologies, followed by quartzites. The Paleozoic
is thicker in the footwall unit (7000-10000 m), but appears
incomplete in the tectonic windows. A strongly competent
horizon of Lower Cambrian sandstones, the Xistral
Quartzite, occupies most of the windows, reaching a
structural thickness of 5000 m after having been folded
and locally repeated by thrusting.
The sedimentary succession was deformed during the
Variscan orogeny. A first deformation episode of east­
verging recumbent folding was followed by ductile and
brittle thrusting toward the east (Figs. 3 and 4). Subsequent
open steep folding allowed the present-day preservation
from erosion of around 10 km of the Mondofiedo thrust
sheet in an open synform. The basal parts of the thrust sheet
outcrop at its front and also surrOlll1ding the tectonic
windows. To the west, the thrust sheet is bounded by the
Viveiro fault, a west-dipping normal fault cutting across the
nappe and its autochthon (Figs. 1, 3 and 4).
Detailed structural analyses of the hanging wall unit and
the basal ductile shear zone can be found in Martinez
Catalan (1985), Bastida et a1. (1986) and Aller and Bastida
(1993). These contributions deal with the geometry of the
allochthonous unit, its macro-, meso- and microstructures,
the synkinematic metamorphism, and the geometry and
kinematics of the basal shear zone. The significance of the
Mondofiedo nappe in the structural evolution of the
Variscan belt of NW Spain is discussed in Perez-Estaun
et a1. (199 1).
In previous works, the only contractional ductile shear
zone identified was at the base of the hanging wall unit (the
Mondofiedo nappe itself), the only extensional structure
identified was the post-nappe Viveiro fault, and the structure
of the tectonic windows was poorly known. Nappe
emplacement was simply viewed as an allochthonous
sheet moving without much internal deformation over the
basal ductile shear zone and sliding along a brittle thrust
fault once relatively high crustal levels were reached.
Contemporaneous erosional denudation was assumed to
account for decompression indicated by the metamorphic
evolution, and extensional deformation was considered to
have occurred essentially in a post-nappe stage (M:artinez
Catalan, 1985).
After a careful review of the hanging wall unit, new
mapping of the footwall unit in the tectonic windows, and a
petrological study of the metamorphic evolution, a complex
picture of ductile shear zones (Figs. 3 and 4), with different
kinematics and partially overlapping in time, has emerged.
The petrological study has been carried out in representative
domains of the nappe and its footwall unit, but these results
are presented in a companion paper (Arenas and Martinez
Catalan, 2003).
3. Shear zones in the hanging waIl unit
The structure of the Mondofiedo nappe is dominated by
east-verging recumbent folds. The geometry of these large
folds suggest a strong flattening, which is corroborated by a
pervasive axial planar cleavage (S). The overturned limb of
the main recumbent fold, the Mondofiedo-Lugo-Sarria
(MLS) anticline, reaches 15 km in the upper parts of the
stratigraphic section (Fig. 4). The recumbent folds reflect an
episode of crustal shortening and thickening, and were
affected by a regional metamorphism of intermediate
pressure, as defined by Miyashiro (196 1), with kyanite­
sillimanite, common in midcrustal levels of many orogenic
belts (Thompson and England, 1984). The metamorphic
zoning, of Barrovian type, includes chlorite, biotite, garnet,
staurolite-kyanite, sillimanite and sillimanite-orthoclase
VIVEIRO
FAULT
43° 30'
l3IJ
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TWO·MICA SYN· TO POST·
KINEMATIC GRANITOIDS
BIOTITIC SYNKINEMATIC
GRANITOIDS
DUCTILE SHEAR ZONES
-
CONTRACT/ONAL
HANGING WALL: BASAL SHEAR
ZONE OF THE MONDONEDO NAPPE
FOOTWALL
EXTENSIONAL
l3IJ
l3IJ
HANGING WALL: UPPER
EXTENSIONAL SHEAR ZONE
FOOTWALl: LOWER
EXTENSIONAl SHEAR ZONE
Cross section
B-B'
AXIAL TRACES OF
LATE OPEN FOLDS
ANTIFORM
SYNFORM
AXIAL TRACES OF
RECUMBENT FOLDS
__0 __0_
ANTICLINE
-.
SYNCLlNE
--
,--
7° 30'
o
10
20 km
I:==�-===-Fig.
3. Map showing the distribution of ductile shear zones in the hanging wall and footwall to the Mondoiiedo thrust and the axial traces of major folds. Faults
1. Stratigraphic and lithological keys are used only for cover and granite massifs.
and unit bOlmdaries allow a comparison with Fig.
Cross section A-A'
BALSA
TECTONIC
SLICE
o
MONSEIBAN
MASSIF
MONDONEDO­
LUGO-SARRIA
ANTICLINE
10
20 km
VILAOUDRIZ
SYNCLlNE
o
BASAL THRUST OF THE
MONDONEDO NAPPE
SE
NW
��
ISOGRADS
MONDONEDO THRUST SHEET
Cross section
8-8'
BIOTlTE
ANTICL/NE
GARNET
STAUROLlTE-KYANITE
SW
NE
SllLlMANITE
SILlIMANITE-ORTHOCLASE
o m -TJ,,,,,,
XISTRAL TECTONIC WINDOW
SllLlMANITE
SllLlMANITE-ORTHOClASE
"OLLO DE SAPO"
ANTICLINE
\
UPPER EXTENSIONAL
SHEAR ZONE
BASAL THRUST OF THE
MONDONEDO NAPPE
DUCTILE SHEAR ZONES
CONTRACTIONAL
HANGING WAll: BASAL SHEAR
ZONE OF THE MONDONEDO NAPPE
NE
SW
FOOTWAll
Om
EXTENSIONAL
D
D
BASAL SHEAR ZONE OF
THE MONDONEDO NAPPE
Fig.
4. Two
HANGING WAll: UPPER
EXTENSIONAl SHEAR ZONE
FOOTWAll: lOWER
EXTENSIONAl SHEAR ZONE
general sections across the Mondofiedo nappe, each shown in two versions. The first depicts the lithostratigraphic units (see legend in Fig.
1), and
the second, the shear zones and the metamorphic isograds. Arrows show movement of contractional (black) and extensional (white) shear zones and faults. For
location, see Figs.
1
and
3.
Letters A-F refer to the location of the P-T paths shown
(Fig. 5). The isograds crosscut the recumbent folds
(Capdevila, 1969), and were deformed by the subsequent
ductile shear zones and cut by the thrust and normal faults
(Fig. 4).
The Upper Proterozoic core (Vilalba Series) of the MLS
anticline has a mean thickness of 6 km along most of the
fold, as deduced from cross-sections constructed by down­
plunge projection of the hinges and limbs of the second-
in Fig.
6.
order folds. To the W and SW, however, the Vilalba Series
becomes progressively thinner, and the fold core is less than
1 km thick (Fig. 4). The thinning of the fold nappe toward its
internal parts is not only reflected in the core of the anticline,
but also in the thickness of the Paleozoic formations on both
limbs and in the width of the metamorphic zones (Figs. 4
and 5). Thinning is due to the superposition of two ductile
shear zones with opposite senses of movement. One of these
7" 30'
-43°30'
[32J
m
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TERTIARY AND QUATERNARY
BIOTITIC POSTKINEMATIC
GRANITOIOS
TWO-MICA SYN- TO POST­
KINEMATIC GRANITOIOS
BIOTITIC SYNKINEMATIC
GRANITOIOS
METAMORPHIC ZONES
D
D
D
D
D
_
o
t» �<H
I! . ;d:
_
,
MONDONEDO THRUST SHEET
CHLORITE
BIOTITE
GARNET
STAUROLlTE-KYANITE
SILLlMANITE
SILLlMANITE-ORTHOCLASE
TECTONIC WINDOWS
CHLORITE-BIOTITE
STAUROLlTE-COROIERITE
SILLlMANITE
SILLlMANITE-ORTHQCLASE
ISOGRADS
MONDONEDO THRUST SHEET
______
BlonTE
.................. GARNET
STAUROLlTE-KYANITE
_._._._.
SILLlMANITE
_•• _••_•••
SILLlMANITE-ORTHOCLASE
TECTONIC WINDOWS
7" 30'
o
10
20 km
t:::::==--==-_
•••••••••
STAUROLlTE-COROIERITE
_._._._.
SILLlMANITE
.
_•• _•• _ .
.
SILLlMANITE-ORTHOCLASE
Fig. 5. Map of metamorphic zones and isograds. Note how the zones narrow in the western part of the nappe, due to the superposition of the upper extensional
shear zone and the Viveiro fault.
is the top-to-the-east basal shear zone of the Mondoiiedo
nappe, which sheared and thinned the overturned limb of the
l\1LS anticline, doubling its cross-sectional length in its
deep parts, where it attains more than 30 km (compare units
deformed in the shear zone with lll1its in less deformed parts
above in cross-section A-A' of Fig. 4). The other shear zone
is a shallow-dipping extensional ductile structure with top­
to-the-west motion, developed in the upper parts of the MLS
anticline (Fig. 4; cross-sections A- A' and B -B').
This section describes both the basal (reverse) and the
upper (normal) shear zones, and demonstrates their
temporal overlapping. The tectonothermal evolution of the
nappe will be outlined along with the structural description.
A detailed petrological description of the metamorphism is
provided elsewhere (Arenas and Martinez CataJan, 2003).
Here, only the key aspects emerging from our study of the
regional and local distribution of metamorphic zones (Fig.
5), and the P-T conditions and evolution will be mentioned.
Three P-T trajectories corresponding to upper, intermediate
and lower parts of the Mondonedo thrust sheet are shown in
Fig. 6, paths A, B and C, respectively. Little control is
available for the prograde paths (dashed), but the pressure
peaks vary from 6 to 11-12 kbar, indicating that the pile of
recumbent folds reached a depth of 38-45 km, and what
i13
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01
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9
8
7
6
5
4
3
2
1
0
�2700��3700��470�570�670�770�870�
T (·C)
GENERAL P·T PATHS FOR THE MONOONEDO NAPPE
" Upper part of the nappe. Biotite and garnet zones
G) Middle part of the nappe. Staurolite-kyanite zone
(9 Lower part of the nappe. Sillimanite and
sillimanite-orthoclase zones
GENERAL P·T PATHS FOR THE FOOTWALL UNIT OF
THE MONDONEDO NAPPE
4j) Hanging wall to the lower detachment. Chlorite­
biotite zone
t:) Hanging wall to the lower detachment.
Staurolite-cordierite and sillimanite zones
C) Footwall to the lower detachment
Sillimanite and sillimanite-orthoclase zones
(!) Deep, ascending thermal source
Fig. 6. P-T paths for the Mondofiedo thrust sheet (grey arrows), its footwall
unit (white arrows),
and deeper crustal rocks llllderlying the lower
extensional shear zone (black arrow). The paths are representative of
different parts of the nappe and its autochthon, which are indicated with
capitals in the cross-sections of Figs.
4, 8
and
13.
Based on Arenas and
Martinez Catalan (2003). Dashed lines represent parts of the paths not based
on petrological data.
presently constitutes the nappe was initially around 20 km
thick. Temperature peaks are close to pressure peaks and
vary from 500 to 700 °c.
3.1.
Basal shear zone of the Mondoiiedo nappe
The basal shear zone is narrow at the frontal parts of the
nappe, where no more than 200 m of sheared rocks crop out.
The shearing overprints earlier structures and developed a
crenulation fabric in slates, very low-grade phyllonites and
cataclasites (Marcos, 1973). Conversely, the thickness of
the basal ductile shear zone attains 3-3.5 km in the internal
parts of the nappe, where it crops out surrounding the two
tectonic windows and also in two domes formed by
interference of orthogonal late open folds in Portomarin
and to the west of Lugo (Figs. 3 and 4). The shear zone is
characterized by a generalized crenulation cleavage or a
new medium-grained schistosity (S2) in the pelites, a
mylonitic foliation in the quartzites (Bastida and Pulgar,
1978), and a mineral lineation roughly striking E-W. Minor
folds with curved hinges, including sheath folds, and S-C
structures are also common. The kinematic indicators
consistently give a top-to-the-east sense of shear. A few
metres of ultramylonites have been found locally at the
thrust surface.
Aller and Bastida (1993) described in detail the basal
shear zone along the coast, to the east of the Xistral tectonic
window, establishing a threefold subdivision according to
microstructural criteria. In the lower levels, mylonites and
ultramylonites are common, and quartz c-axis fabrics are of
the small-circle type (Schmid and Casey, 1986). This
indicates strong shearing at low temperature conditions, and
represent late stages of thrusting, contemporaneous with
greenschist facies retrogradation. The middle part of the
shear zone contains blastomylonites, and the quartz c-axis
fabrics are of the monoclinic incomplete single girdle and
type I crossed girdle, suggesting deformation at higher
metamorphic conditions, mostly in the amphibolite facies.
The upper part includes non-mylonitic quartzites and
quartzites with small-circle type and type I crossed girdle
which, together, suggest lower temperatures than in the
central parts.
The internal parts of the basal zone suffered the highest
P-T conditions, and also the most intense retrogradation.
After the Barrovian-type metamorphic event, nappe empla­
cement began along the ductile shear zone. Sillimanite grew
synkinematically in the deeper parts of the nappe, while
andalusite porphyroblasts developed in its upper parts. All
the P-T paths show a decompression, which was greater in
the deeper parts (compare paths A and B with path C in Fig.
6 and their locations in Fig. 4). The decompressive paths
were, at an initial stage, close to isothermal for the lower
parts (Fig. 6, path C), suggesting a quick exhumation
(Thompson and England, 1984), a feature commonly
associated with tectonic denudation. Conversely, the late
stage shows cooling accompanied with slight decompres-
sion. This corresponds to a relatively thin thrust sheet
(roughly 12 km, indicated by the 3 kbar at its base; see path
C in Fig. 6) being emplaced at relatively shallow levels and
undergoing only slight denudation.
The presence of deformed Variscan granitoids is another
important feature of the basal shear zone. The Sarria,
Hombreiro, Santa Eulalia, Monseiban (Fig. 3) and other
minor massifs partly intruded into the shear zone, and were
sheared toward the east (Fig. 4). Martinez Catalan ( 1983)
and Aranguren and Tubfa (1992), described the microstruc­
tures and estimated the temperature conditions of defor­
mation, which were close to the solidus temperature for
granite. The fact that shearing closely followed their
emplacement points to their synkinematic character.
3.2.
Upper extensional shear zone
In the western region of the nappe, the stratigraphic
sequence, the recumbent folds and the metamorphic zones
thin gradually to the west (Figs. 4 and 5). This is a region
where a subhorizontal crenulation cleavage or a new
schistosity was developed, as well as an E-W mineral
lineation. This region contrasts with wide areas to the east,
which are structurally lower, and where only the first
cleavage (SI) is present. There is clearly a zone of
deformation superimposed on the normal limb of the MLS
anticline. Its maximum thickness is estimated at 2 km, and
decreases progressively to the south of Oural (Fig. 3).
Asymmetric pressure shadows, developed around pre- and
synkinematic porphyroblasts, indicate a top-to-the-west
sense of shear.
The shear zone deforms the recumbent folds and the
Barrovian metamorphic zones, and is considered to post­
date them. However, porphyroblasts of kyanite, staurolite
and andalusite grew in the shear zone synkinematically with
the second cleavage. This implies that the P-T conditions
were still high, and suggests that motion in the shear zone
began during the early stages of nappe emplacement.
Because the P-T conditions were greater there than at the
base of the thrust sheet in later stages of emplacement, the
upper shear zone probably finished its activity before
the end of the thrusting process.
An additional criterion for the relatively early timing of
the upper shear zone is provided by the Sarria massif: its
upper part intruded the shear zone after shearing had ceased,
because the granite is undeformed there. However, the
massif intruded in the nappe while it was still being
emplaced, and was deformed within the basal shear zone
(Figs. 3 and 4).
The thinning of the metamorphic zones and the
apparently down-dip motion (top-to-the-west) suggest a
normal character for the shear zone, being equivalent to a
broad extensional detachment. In terms of thermobarome­
try, this is reflected in the isothermal, decompressive portion
of the P-T paths followed by the deep parts of the
Mondoiiedo nappe (Fig. 6, path C). Its kinematics might
imply the tectonic extrusion of the MLS anticline from the
root zone, in a way comparable with that proposed by
Dietrich and Casey (1989) for the Helvetic nappes.
However, the :MLS anticline is different to the Helvetic
case, as the upper shear zone has a sense of motion opposite
to that of the basal one. Alternati vely, the upper extensional
shear zone may be a consequence of gravitational collapse,
in response to gravitational gradients created by orogenic
topography.
The upper shear zone was later overprinted by the
Viveiro normal fault. Because of the spatial coincidence of
both structures (Fig. 3), they have been considered
associated (Martinez Catalan, 1985; Martinez et aI., 1996).
However, in the light of the new data, they are considered
here to be separated in time: the upper ductile shear zone
moved during nappe emplacement, whereas the Viveiro
fault cuts the Mondofiedo thrust sheet and its autochthon.
4. Shear zones in the footwaIl unit
10 the western part of the Mondoiiedo nappe, two
families of open folds, longitudinal (N-S to NE-SW) and
transverse (NW-SE to E-W), interfere allowing the
footwall unit to crop out (Fig. 3). A longitudinal antiform
is essentially responsible for the large northern tectonic
window (Xistral), and the same, when interfering with an
E-W transverse antiform, causes the southern small
window (Monte Carballosa). This section describes the
different structural units exposed in these tectonic windows,
together with the main ductile shear zones identified (Fig.
7). Focusing on the Xistral window (except when specifi­
cally stated), this section uses longitudinal and transverse
cross-sections (Fig. 8) to study the complex pattern of
compressional and extensional structures contemporaneous
with thrusting. A three-dimensional sketch showing the
relationships between the main compressional and exten­
sional structures is shown in Fig. 9.
The Xistral Quartzite (Fig. 2) was strongly folded prior to
nappe emplacement. When not affected by additional
deformation, cross-bedding indicates the way-up direction,
permitting the mapping out of the large folds. 10 these cases,
the SI foliation is axial planar to the folds, a stretching
lineation is not thoroughly developed and textures are not
mylonitic. Conversely, when affected by subsequent shear
zones, the quartzites were mylonitized, a pervasive
elongation of quartz grains developed, commonly striking
E-W (Fig. 7), and cross-bedding is poorly preserved and
difficult to interpret. In addition, new minor folds developed
locally.
Relics of the basal shear zone of the Mondoiiedo nappe
have been preserved in the footwall unit, and several
tectonic slices exist close to the thrust fault. Furthermore, a
strain gradient to the SE is identified in the Xistral Quartzite
in the south, and interpreted as a contractional shear zone
converging upward into the Mondofiedo basal thrust.
o
20 km
10
LOWER
DETACHMENT
BASAL THRUST OF THE
MONDONEDO NAPPE
L OWER
DETACHMENT
E:;::':::=o-. CADRAM6N
SLICE
LOWER
DETACHMENT
1< <1
XISTRAL QUARTZITE
DUCTILE SHEAR ZONES
MONDONEDO THRUST SHEET
Cross
section
IV-IV'
D
-
43020'
5,
FOLIATION OUTSIDE
THE SHEAR ZONES
N
0, AXES AND l,
INTERSEC·
TION LINEATION OUTSIDE
THE SHEAR ZONES
N
UPPER EXTENSIONAL SHEAR ZONE
BASAL SHEAR ZONE OF
THE MONDONEDO NAPPE
XISTRAL TECTONIC WINDOW
BASAL SHEAR ZONE OF
THE MONDONEDO NAPPE
MINERAL LINEATION
INSIDE
THE SHEAR ZONES
D
LOWER CONTRACTlONAL
SHEAR ZONE
LOWER EXTENSIONAL SHEAR ZONE
n = 80
MYLONITIC FOLIATION IN THE
LOWER CONTRACTIONAL
SHEAR ZONE
MYLONITIC FOLIATION IN THE BASAL SHEAR ZONE OF THE
MONDONEDO NAPPE IN THE FOOTWALL TO THE THRUST:
BALSA SYNFORM
MURAS SYNFORM
CADRAM6N SLICE
N
Fig.
N
N
N
7. Map shO\ving the distribution of ductile shear zones in the Xistral tectonic window, and diagrams depicting the attitude of foliation and lineation (lower
hemisphere, equal area stereographic projection).
Cross
section
RUA EXTENSIONAL
SHEAR ZONE
Cross section I-I'
V-V'
SAN CIPRIAN
MASSIF
VIVE/RO
MASSIF
NW
BURELA
SLICE
Cross section 11-11'
V1VEIRO
FAULT
E
LITHOLOGIES
..
Om
BASAL SHEAR _
_
--T"
ZONE OF THE
MONDONEDO NAPPE
�
�
�
�
SE/XO
U
A,;iii'E'/T BLANCO
-E,
SLICE
Cross
section
CADRAMON
SLICE
CIJl
V-V'
Cross section Ill-Ill'
NW
POSTKINEMATIC GRANITOIDS
TWO-MICA SYNKINEMATIC
GRANITOIDS
BIOTITIC SYNKINEMATIC
GRANOTOIDS
ARMORICAN QUARTZITE
XISTRAL QUARTZITE
,
Om --.----:
UPPER
EXTENSIONAL
SHEAR ZONE
FAULTS
THRUST FAULTS
Cross section IV-IV'
LOWER CONTRACTlONAL
SHEAR ZONE
NORMAL FAULTS AND
EXTENSIONAL DETACHMENTS
E
V/VEIRO
FAULT
EARLY NORMAL FAULTS
om- ��
��
Cross section V-V'
BASAL SHEAR ZONE OF
THE MONDONEDO NAPPE
CADRAMON
SLICE
BALSA SLICE
BASAL THRUST OF THE
MONDONEDO NAPPE
NE
SW
LOWER CONTRACTlONAL
SHEAR ZONE
o
SLICE
10
20 km
LOWER EXTENSIONAL
SHEAR ZONE
DETACHMENT
RUA EXTENSIONAL
SHEAR ZONE
,
Fig.
8.
Geological sections across the Xistral tectonic window showing the ductile shear zones and faults. For location, see Fig. 7. For legend of the shear zones,
see Figs.
3 and 4. Letters C-F refer to the location
+ ) S2 in the shear zones.
of the P-T paths depicted in Fig. 6. Thin dashed lines show the attitude of the main foliation: Sl outside the
shear zones, and (Sl
The main extensional structure is a detachment situated
at the bottom of the quartzitic formation and below it (Figs.
7 -10). In addition, horizontal extension affected the
quartzite in both longitudinal and transverse directions.
The E-W extension created faulted blocks that underwent
domino-style rotation (Fig. 8, cross-section I-I'). The N-S
extension gave rise to transverse normal faults cutting
across the footwall unit (Fig. 8, cross-section V-V', and
N
f
ROOT
MLS AN ""UN
>
Tt--
•.O'VER DETACHMENT
UPPER EXTEN510NAL
SHEA R ZONE
--��--6LUGO-SARRIA
:��:�����fih�
(MLS)
ANTICLINE
Fig.
9.
Three-dimensional sketch showing the relationships between the
main contractional and extensional struchues. The upper part of the figlUe
depicts the crosscutting relationships between folds and faults, whereas the
lower block shows the two main extensional ductile shear zones (light grey,
the notth. west of Cadram6n (Fig. 7). also occurs on the
southern limb of another very open transverse synform.
The mylonitic foliation in the shear zones defines the
saddle form of the fold and is at a low angle to thrust faults
within the window. which are also affected by the fold
interference. This contrasts with the SI foliation below,
which is at nearly 900 to these thrust faults (Fig. 8. cross­
section IT-II'). Stretching lineation. marked by elongation
of quartz grains. varies from E-W to NW-SE. and shear
criteria indicate top-to-the-east kinematics. The quartzite
shows a late-kinematic generalized grain growth (Fig. 1 1c)
and a quartz c-axis fabric (Fig. 12. sample PG-77) discussed
in a separate section.
The structural position, always directly lll1derneath slices
of the Mondonedo nappe (see below). suggests that the
mylonitic remnants are relics of the Mondonedo basal shear
zone, preserved in its footwall. In contrast to the hanging
wall unit, the maximum thickness of the shear zone is 1 km
in the footwall, a value attained only in the hinge zone of the
Muras-Balsa radial synform and decreasing progressively
to zero in the limbs (Fig. 8. section V-V'). It seems as if the
shear zone was being folded in the radial synform and, at a
given moment of the folding process, was cut by the
Mondonedo thrust fault (Fig. 10). The shear zone itself was
cut and repeated by late-stage thrust faults. giving rise to
two quartzitic imbricates (the Acibreiro and Seixo Blanco
slices; Figs. 7 and 8). We will turn back to these points later.
when describing the N-S extension.
4.2.
Lower contractional shear zone
'transparent'). The contractional ductile shear zones are not shown.
Fig. 9). All these structures were active during different
stages of nappe emplacement.
The metamorphic evolution of the footwall unit is very
different from that of the hanging walL It is better described
by its low-pressure conditions and by a strong thermal
gradient (Fig. 6. white arrows). and is similar to the low-P
intermediate type of Miyashiro (1961). characterized by
parageneses with andalusite-staurolite-cordierite. An epi­
sode of vigorous heating affected most of the Xistral
window and is clearly identified in the quartzites, where it
gave rise to a pronounced grain growth, initiated in the
advanced stages of deformation (Fig. 1 1).
Basal shear zone a/the Mondofiedo nappe in the
footwall unit
4.1.
Remnants of a ductile shear zone have been found in the
upper parts of the Xistral Quartzite, in two contiguous
outcrops that surround a NW-SE-trending synform
between Muras and Balsa (Figs. 3 and 7). The structure
here is defined by a saddle-fold geometry formed by
interference between the transverse synform and the large
longitudinal antiform. A third outcrop. a few kilometres to
In the south of the Xistral window and in the Monte
Carballosa window, the quartzite became mylonitized to the
east, below a less-deformed zone where the recumbent folds
can be easily mapped (see Figs. 3 and 7). In the Xistral
window, the mylonitic foliation is folded in an interference
dome (see stereographic projection in Fig. 7). but is oblique
to the Mondonedo basal thrust (Fig. 8. cross-section IV­
IV') in such a way that unfolding the late longitudinal
antiform, it would dip some 30° more to the west than the
thrust itself. In fact, both shear zones converge to the south
of Balsa (Fig. 7). This feature. together with the kinematic
criteria indicating top-to-the-east shearing, supports the
interpretation of the shear zone as having been a wide
ductile thrust.
The mylonitic texture of the quartzites is well marked by
quartz ribbons bounded by flat and parallel muscovite
crystals. The quartz grains show a late- to post-kinematic
generalized grain growth typical of high temperature
annealing (Wilson. 1973; Bouchez and Pocher. 19 8 1) in
the lower and middle parts of the shear zone in the Xistral
window (sample PG-62; Fig. lIb). but not in the upper part
(sample PG-59; Fig. 1 1a). nor in the Monte Carballosa
window where the metamorphic grade remained low
through the entire history of deformation. Paths E and D
of Fig. 6. depict the metamorphic evolution of points of the
BASAL SHEA R ZONE OF
THE MONDONEDO NAPPE;
HANGING WA"LL
:..:==
FOOTWALL -
:
=: .
MONDONEDO NA PPE
.. .. . . . .. .. .. .. .. .. . .... ..... ..... .... ...............................··.b..............
.
.
.
............................... .. . .
·
.
:.:... .... .'� '� ' �
. ..... ....... "'
.
'.
... .
" .;'.;'. " . . � ..� ..�,;;.�,;;.�,;;.� ..� ......�.
....
.....�.:.. :...�,;;.�,;;.�,;;.�,;;.�,;;.�,;;.� ..� ..� ..� ..� ..�.. . ...� ..�
10
o
!
I::::::::�
XISTRAL QUARTZITE
_'L-_"'�
ACTIVE THRUST
...
ABANDONED
--
...
-
..•......••
THRUST
FUTURE THRUST
EXTENSIONAL
DETACHMENT
••• 1 •••••• 1 •• ,
FUTURE
DETACHMENT
ACTIVE NORMAL
FAULT
FUTURE NORMAL
FAULT
NI
S
XISTRAL TECTONIC WINDOW
MONTE CARBALLOSA
TECTONIC WINDOW
Fig.
BASAL THRUST OF THE
SW MONDOIVEDO NA PPE
TE£T�'i§icNE
LOWER EXTENSIONAL
SHEA R ZONE:
HANGING WALL
_
FOOTWALL
_
-'
10. Footwall sequence diagram showing the evolution of the lower unit of the Mondofiedo thrust in a section normal to the transport direction (section V-V' of Fig. 8). The
progressive thickeillng of the basal
shear zone of the Mondofiedo nappe (dark grey) from (a) to (d) tries to reflect the fact that moreintemal parts of the nappe (where the basal shear zone was wider) were reaching the section, thus reflecting thrust
motion contemporaneous with the N-S extension.
0.80
mm
•
-
Fig.
11.
l\1icrophotographs of mylonitized quatrzites in the shear zones of the Xistral tectonic window, showing the effect of heating on grain size. (a) Original
fine-grained fabric of the lower contractional shear zone preserved aTOlmd Vilapedre, in the southern part of the Xistral window (Fig.
7), and
corresponding to
the upper part of the stmuolite- cordierite zone. (b) Mylonitic fabric of the lower contractional shear zone at the limit between the sillimanite and stalUolite­
cordierite zones. Grain growth is very pronOllllced and evident when comparing with the previous photograph (note the different scales). (c) Fabric of the basal
shear zone of the Mondofiedo nappe in the footwall unit inside the sillimanite zone. (d) Fabric of the quartzites adjacent to the lower detachment and to the
sillimanite-orthoclase zone, showing generalized and exaggerated grain growth. Note the parallel inclusions of white mica corresponding to an older, fine­
grained foliation. Crossed polarizers. The struchual position of the samples is shown in Fig.
footwall unit equivalent to the sampling sites ofPG-62 and 59. respectively. and Fig. 12 (cross-section IV-IV') shows
the structural position and quartz c-axis fabric of both
samples.
4.3.
Lower extensional shear zone and detachment
The base of the Xistral Quartzite delineates a N-S
antiform around Viveir6, a hamlet in the centre of the
tectonic window (not to be confused with the village of
Viveiro, 20 km to the north, on the coast, and which gives its
name to the Viveiro fault; see Fig. 7). The limit is very
smooth, when one considers the fact that the quartzites are
strongly folded. This basal contact is not a stratigraphic one.
but a detachment fault, associated with which there is a
lOO-350-m-thick mylonitic shear zone affecting the base of
the quartzites above. The mylonites include a new
generation of minor recumbent folds with east vergence.
These and the asymmetry of sigmoidal boudins indicate a
top-to-the-east shearing. As is usually found in the Xistral
tectonic window, the quartz-mylonites underwent grain
growth. though here it is even more exaggerated (sample
FO-33 in Fig. l l d). Individual quartz grains may attain
1 2.
10 mm along their long axis. which commonly (but not
always) is parallel to the apparent mineral lineation of the
rock. marked statistically by the long axes of the quartz
grains. The mylonitic foliation is parallel to the detachment.
but the layering and the SI foliation in the Xistral Quartzite
above the shear zone are inclined between 30 and 90° to it
(Fig. 8. cross-sections I-I' to IV-IV').
High-grade paragneisses of the sillimanite and the
sillimanite-orthoclase zones occur below the detachment,
constituting its footwall. A high-temperature and low­
pressure penetrative foliation. roughly parallel to the
detachment, characterizes its footwall, which is also
penetratively deformed. Numerous granitic injections
evidence partial melting, and the metamorphic associations,
characterized by the absence of kyanite and the scarcity of
garnet, indicate that high temperatures were associated with
relatively low pressures (Fig. 6. path F). No low-T relics are
found in the footwall to the detachment, whereas they are
common in the quartzites above, where parallel inclusions
of very small size (mostly white mica) in large new quartz
grains, point to a first mylonitic stage developed at low
grade conditions (Fig. l Id). Also. pelitic horizons inside the
hanging wall quartzites are low- to medium-grade schists,
�����;;��;:=::-
Cross section I-I'
_
_
�::::,,-£:::::
: ::
LOWEREXT
E NSIONALSHEAR
E O TH E
A
B SA LSH
- EA R Z O
pNp
'F
M ON O ONE O O N A
ZONE IN THE HA NGING WALL
E
N T HE
TOT
HE LOWERDET
ACHME NT
FOOTWAL
LTOTHE THRUST
PG-77
Srn: 142"165" SW
Lm:
E
LOWEREX
T
E NSIONALS HEAR lONE IN THE HA NGING
THE LOWERDET
ACHME NT
WAL
LTO
Cross section
n=
111·111'
SE
236
BASALSHEAR ZONE OF THE
M OND ONED O NAPPE IN THE FOOTWAlLTOTHE THRUS T
PG-79
Srn: 130<'130° NE
: 75°125° E
sw
PG-7S
Srn : 150"120"
lm: 86°/18° W
y-.::::::---_ E
Lm
E
VIVEI
R OSHEAR ZONE
10
20 km
,""===::::JI___
t::==::::JI___'
PG�6
--"
��
�
Srn: (J"170· W
Lm: 125'/66' W
w
Cross section IV-IV'
E
LOWERCON T
RACT
IONALSHEAR ZONE
::o:
s =---"''-''
:
''
'''/'-
·0
: "1:: 30
:: :=O
����2
95°{13° W
lm:
w
E
--.rlLmf--�� ---I Lm
SW
Cross section V-V'
A�
I
NE
N SW
����\ ,���
������
���
��
-
�
....... �
LOWEREXT
E NSIONALSHEAR ZONE IN THE HA NGING WALL TO THE
LOWER DET
ACHME NT:S OUTHER N L
I
M B OF THE BAL
SA SYNCLINE,
WITH N -S MINERAL LINEA TION
PG·76
Srn: 145"/65" SW
Lrn: 160"'29" S
S
Fig.
12.
samples.
-
S
Quartz c-axis fabrics from the Xistral Quartzite mylonitized
N
in
the different shear zones. Cross-sections of Fig.
8
are used to locate the analysed
which were later heated to the sillimanite zone (Fig. 6, path
E), but never transformed into paragneisses.
These petrological constraints are used as evidence
indicating a jump in metamorphic grade across the
detachment, with the omission of part of the metamorphic
zoning, pointing to an extensional movement (Wheeler and
Butler, 1994), coupled with heating of its upper unit with
heat transmitted from the lower unit. Yet the P -T paths for
the upper and the lower units (Fig. 6, paths E and F,
respectively) run almost parallel. Both are nearly isobaric
prograde trajectories, of the type commonly developed in
the hanging wall to large extensional detachments (Escuder
Viruete et aI., 1994, 1997). Considering the P-T path of the
lower unit (Fig. 6, path F), the possibility arises that this
footwall represents in fact the upper part of a deeper zone of
crustal thinning and stretching, whose lower part consist of
hot material ascending inside the crust.
A series of synkinernatic granites, granodiorites and
locally, tonalites, associated with ultrarnafic rocks, intruded
the nottbern part of the Xistral tectonic window (Figs. 1, 3, 7
and 8), and are collectively described as the Viveiro massif
(Galan, 1987). These rocks were deformed, acquiring a low­
dipping foliation parallel to the regional high-T and low-P
foliation of their country rocks. Galan (1987) and GaJan
et al. (1996) studied these rocks, concluding that the
ultrarnafics have a mantle provenance, and the tonalites and
granodiorites include mantle components.
The geological map NW of Viveiro (Fig. 7) shows that
the lower detachment joins the Mondoiiedo basal thrust to
the west (Fig. 8, section 11-II'). We assume that the thrust
was inclined to the west, but depending on how much, the
lower detachment would have had an original dip either to
the east or west. On purely geometrical grounds, it might be
equivalent to either a normal or a reverse fault. However,
because the omission of part of the metamorphic zoning
suggests it is an extensional detachment, and shear sense
was to the east, we infer it to have originally dipped to the
east.
The lower detachment, the associated narrow hanging
wall shear zone and the widely deformed paragneisses and
igneous rocks below are collectively termed the lower
extensional shear zone (Fig. 8). A related structure occurs in
the northern part of the Xistral window, west of Rua, where
two blocks with west-dipping foliation, are cut by an east­
dipping ductile shear zone with top-to-the-east movement
(Fig. 8, section I-I'). Even though both blocks are made up
of the Xistral Quartzite, the lower one is in the sillimanite
zone, whereas the upper one is greenschists facies (Fig. 5).
This points to the extensional character of the Rua shear
zone, which joins the Mondonedo thrust fault to the south
(Fig. 7).
The Rua extensional shear zone seems dynamically
linked to the lower detachment. Its throw is not necessarily
large, as the thermal gradient associated with the lower
detachment is high and the sillimanite and chlorite-biotite
zones were probably not far from each other. The most
striking feature of this shear zone is the high angle it makes
to the foliation in the two blocks it separates. In both the
Mondofiedo thrust sheet and its footwall unit below the
Xistral Quartzite, the bedding and the foliation dips are
generally low and, more important, at low angles to the
attitude of the thrust fault (Fig. 4). This is not the case in the
Xistral Quartzite, however, where the bedding, the SI
foliation and the axial surface of first-phase folds dip
between 45 and 90°, except inside the ductile shear zones
(Fig. 8). In most of the eastern part of the Xistral window,
the first foliation makes an angle of nearly 900 to the thrust
fault (Fig. 8, sections 1-1' to m-11I'). Also, in both flanks of
the N-S antiform at Viveiro, the planar fabric in the Xistral
Quartzite is parallel to its lower limit only close to it, but
above the shear zone, bedding and foliation quickly rotate,
becoming perpendicular to the lower detachment (Fig. 8,
section II-11').
If the high angle between the foliation and either the
thrust or the lower detachment is an original feature, the first
folds, usually recumbent, would have been vertical in the
Xistral Quartzite. The other possibility is that this angle
resulted from subsequent rotation, and the Rua extensional
shear zone may have allowed the rotation to proceed by
domino-style boudinage. This type of boudinage needs
weak, ductile layers on both sides to accommodate the
rotation of the rigid blocks. The hot footwall unit of the
lower detachment may have worked as the lower one of
such ductile layers, whereas the basal shear zone of the
Mondofiedo nappe would have been the upper one. At least
two blocks, separated by the Rua extensional shear zone,
rotated counterc1ockwise. Another block may have existed
to the west of Viveiro if the antiform there was nucleated by
boudinage. Because folds form at the neck of boudins, the
antiform may have resulted from the E-W individualization
of a new block during E-W extension. This would explain
its nearly N-S attitude, somewhat different from the NE­
SW strike of the tectonic window and of the late open folds
in the northern part of the Mondofiedo nappe (Fig. 3).
4.4. N-S extension and the thrust slices
Megaboudins other than the blocks described in the
previous section also developed during nappe emplacement,
but related to longitudinal stretching of the Xistal Quartzite.
The lower detachment and the basal shear zone of the
Mondofiedo nappe are commonly 2000 m apart, but nottb of
Balsa, they join each other (Fig. 7), and this occurs in the
only area where N-S stretching lineations are seen in the
Xistral Quartzite (Fig. 7; N-S maximum in the stereoplot
showing mineral lineation inside the shear zones).
Elongation is normal to the fold axis in the inner arc of
the Balsa sync1ine, a feature compatible with the sync1ine
being the neck of a megaboudin. This interpretation
explains why the shear zones that developed in the upper
and lower parts of the Xistral Quartzite, approach each other
in this area (Fig. 8, section V-V', below the Balsa slice),
and is illustrated in Fig. lOb.
The subsequent development of E-W-trending normal
faults confirms the N-S extension. Inside the Xistral
window, three of these faults (thick lines in Figs. 7 and 8,
section V-V') crosscut the footwall unit but not the
Mondoiiedo thrust sheet. The most prominent of these
faults, west of Cadrarn6n, cuts the Seixo Blanco slice
(preserved only in the downthrown southern block) but is
cut by the Mondofiedo thrust fault. The E-W fault appears
folded by late longitudinal folds, a feature that can be
appreciated in the map (Fig. 7) and in an outcrop where a
fault-related catac1asite appears folded with a steep axis.
Many other faults have been mapped, but most of them are
late structures, post-dating the longitudinal folds. Conver­
sely, these three are early normal faults pre-dating the latest
stages of thrusting.
The N-S extension, though of limited extent, had an
important imprint in the present configuration of the
Mondofiedo nappe and its footwall unit. The necks of the
boudins and the fault-related grabens have preserved slices
of the Mondofiedo nappe that were abandoned in these
structural depressions by newly formed faults above. This is
the case of the Balsa and Cadrarnon tectonic slices (Figs.
7 -9). Conversely, boudins and horsts were truncated and
quartzitic slices, such as Seixo Blanco, were incorporated
into the thrust sheet.
It has been a common practice in thrust tectonics to use
hanging wall sequence diagrams (e.g. Harris, 1970; Elliott
and 10hnson, 1980) to show the evolution of allochthonous
units. The diagrams are successive longitudinal cross­
sections showing how different units are being incorporated
into the hanging wall unit of thrust systems propagating
sequentially in a piggy back mode. In the case of the
Mondoiiedo nappe, several imbricates were abandoned
below the currently active thrust fault, so that a footwall
sequence diagram (Fig. 10) seems appropriate to show the
development of thrust slices in relation to N-S extension.
Fig. 10 illustrates both the creation of the Balsa slice after
development of the Balsa synform, and how the Seixo
Blanco slice developed by truncation of a megaboudin
North of the Balsa synfom (a-c), itself then being truncated
by an early normal fault (c and d) and lately overprinted by
the latest Mondofiedo basal thrust fault (d). In addition, to
show how the slices developed sequentially and how they
were preserved by the creation of new faults above, Fig. 10
explains the origin of the transverse open folds: they
correspond either to the necks of rnegaboudins, or are
'forced' folds above horsts and grabens bounded by the
early E-W normal faults in the footwall unit. The southern
limit of the Xistral window and the original north and south
boundaries of the Monte Carballosa window, were probably
early normal faults, affecting the Xistral Quartzite but not
penetrating up into the Mondofiedo nappe. The thrust sheet
adapted to this evolving footwall structural topography, and
the normal fault planes became part of the thrust surface,
acting as lateral ramps during the last stages of thrusting.
4.5.
Quartz c-axis fabrics
The shear zones of the Xistral window share a common
quartz c-axis fabric: a type I crossed girdle (Lister and
Williams, 1979; Schmid and Casey, 1986), usually
orthorhombic (Fig. 12). Often, there is a central part oblique
to the rnylonitic foliation and sometimes, the oblique central
part of the crossed girdles, or the more populated of the two
girdles, may be taken as kinematic indicators. In all the
cases, however, shear sense has been checked with
macroscopic kinematic criteria, such as sigmoidal boudi­
nage and fold vergence.
The fabrics are similar to those modelled by Lister and
Williams ( 1979) for simple shear sometimes combined with
coaxial deformation, and correspond to plane strain
ellipsoids according to Lister and Hobbs (1980). Samples
taken outside the shear zones do not show clear crystal­
lographic fabrics.
To test the influence of the grain growth recorded by the
quartzites inside the sillimanite zone, separate diagrams
were plotted for small and large grains of the same thin
sections. Large quartz grains show microinclusions, mostly
of white mica grains, parallel and closely spaced (Fig. l Id).
They are relics of an earlier fine-grained foliation and, when
the grain growth is not exaggerated (Wilson, 1973), that is,
not all of the grains reach large sizes, small quartz grains do
not include white mica (Fig. l lc) and are viewed either as
preserved relics of the pre-heating stage or as recrystallized
new grains.
As can be seen in Fig. 12, crystallographic fabrics are
similar for both grain sizes, which suggests that this type of
c-axis fabric existed before the quartz grains grew.
Behrmann and Platt (1982) obtained similar diagrams in
rocks deformed at low-grade conditions, between 300 and
400 °C. It is quite clear that mylonites developed at
relatively low temperatures and were subsequently heated.
But it seems that heating was accompanied with further
ductile deformation. The largest grains post-date myloniti­
zation in the lower detachment (Fig. 11d) so that, there,
grain growth continued after mylonitization had ceased.
However, the large quartz grains often define the mineral
lineation. This is the case of the lower contractional shear
zone (Fig. 1 1b), the basal shear zone of the Mondofiedo
nappe in the footwall unit (Fig. 11c), the lower detachment
and the hinge zone of the Balsa syncline, the supposed
megaboudin neck of the N-S extension. In these cases, the
shape fabric of large quartz grains fits the c-axis fabric,
suggesting that deformation was active during heating.
In sections where the Xistral Quartzite can be traced from
low-grade to the sillimanite zone (Fig. 5), the metamorphic
grade increases downward, and grain growth occurs only in
the staurolite-cordierite and sillimanite zones, being
generalized and pronounced only in the latter. For instance,
NW
MONDONEDOLUGO-SARRIA
ANTICLINE
SE
VlLAOUDRIZ
SYNCLlNE
a
BlonTE
GARNET
STAUROLlTE-KYANITE
SllllMANITE
SILLIMANITE-ORTHOCLASE
BASAL SHEAR ZONE OF
THE MOHDOREDO NAPPE
b
BASAL THRUST OF THE
MONDOREDO NAPPE
o
50 km
c
�
CI2J
.....
D
D
D
�
D
-..... -D
0
f§d
ARMORICAN
QUARTZITE
XISTRAl
QUARTZITE
VILALBA
SERIES
SYNKINEMATIC
GRANITOIDS
INTRUSIVE MAFIC AND
ULTRAMAFIC ROCKS
MONDONEDO
NAP PE
FOOTWALL TO THE
MONDONEDO NAPPE
ACTIVE DUCTILE
SHEAR ZONE
INACTIVE DUCTILE
SHEAR ZONE
PARTIALLY MOLTEN
GNEISSES
ACTIVE THRUST
FAULT/SHEAR ZONE
ACTIVE NORMAL
FAULT/SHEAR ZONE
the original fine-grained fabric of the lower contractional
shear zone (Fig. 11a) has been preserved aronnd Vilapedre,
in the southern part of the Xistral window (Fig. 7). This
indicates that grain growth was induced by heat transferred
from below, and we have seen that heating was coeval with
the activity of the lower detachment, which we suggest is an
extensional structure. Furthermore, the simultaneous shear­
ing and grain growth in the Mondoiiedo shear zone (Fig.
1 1c) demonstrates temporal overlapping (or alternance)
between ductile thrusting and motion of the lower detach­
ment, that is between contractional and extensional
structures.
5. The Viveiro fault
A normal fault limits the Mondoiiedo thrust sheet to the
west, crosscuttiog its hanging wall and footwall units (Figs.
1, 4, 7 and 9). The brittle fault dips between 40 and 600 west,
and has an associated ductile shear zone, a few hundreds of
metres thick, that has not been shown in the maps and cross­
sections for the sake of clarity. There, the previous regional
foliation, either the SI or the S2 of the Mondofiedo basal or
upper extensional shear zones, appears crenulated by a new,
subhorizontal cleavage. 11inor recumbent folds with west
vergence and weakly curved hinges, and S-C or ECC
microstructures (Platt, 1984), indicate a top-to-the-west
motion, with a slight right-lateral component (Martinez
Catabin, 1985; Martinez et aI., 1996). Where the Xistral
Quartzite was involved in the shear zone, a mylonitic
foliation developed, whose c-axis fabric shows a main
girdle oblique to the foliation, supporting the top-to-the­
west sense of shearing (Fig. 12, sample PG-86).
The upper extensional shear zone and the Viveiro fault
coincide spatially (Fig. 3), but the metamorphic evolution of
the Mondofiedo thrust sheet demonstrates that they were not
simultaneous. The upper shear zone developed during early
stages of nappe motion, when the thrust sheet was thick and
glided above the ductile basal shear zone, whereas the
Viveiro fault cuts the thinned thrust sheet, the brittle thrust
fault, and the footwall unit, indicating that it formed when
nappe motion had ceased. The Viveiro fault overprints the
earlier shear zone, which was dragged dO\vnward to the west
by the more steeply-dipping fault (Figs. 4 and 9). It is
possible that the fault used parts of the pre-existing weak
shear zone to nucleate and develop.
The shear zone associated with the Viveiro fault
Fig.
13.
developed under low-grade metamorphic conditions. Kya­
nite and staurolite presently fOlll1d around the fault grew
during motion of the upper extensional shear zone, when the
preserved upper levels of the Mondofiedo nappe were deep
enough to fall into the kyanite field, and their decompressive
P-T path went through this field for a large part of its
trajectory (Fig. 6, path A). Martinez Catalan et al. (1990)
suggested a throw of 10-12 km for the Viveiro fault,
considering the metamorphic gap: it separates the chlorite
zone to the west, from the sillimanite zone to the east (Figs.
4 and 5). A more precise estimation of the vertical offset,
based on thermobarometry, was given by Reche et al.
(1998) as between 4 and 5 kbar, roughly equivalent to 1519 km. This throw is viewed as the result of the two dip-slip
motions: that of the upper extensional shear zone and that of
the Viveiro fault itself. A dip-slip of 5-6 km is more
reasonable for the Viveiro fault alone in the north (Fig. 4,
cross-section A-A'). To the south, the fault slip decreases
progressively, being less than 1 km in the area of ficio (Fig.
3).
6. Absolute and relative timing and structural evolution
The structural history of the Mondoiiedo nappe and its
footwall is graphically shown in Fig. 13. The different
stages are based on overprinting criteria and crosscutting
relationships described in previous sections, which will be
recalled to justify the structural evolution. The relative
chronology, bracketed by published isotopic age data
discussed below, is summarized in Fig. 14.
The age of the first deformation phase is constrained by
40Arp9Ar whole-rock geochronology on the SI foliation in
adjacent areas (Dallmeyer et aI., 1997). Toward more
internal zones, the first cleavage was dated at
359.3 ± 0.2 Ma in the western limb of the '0110 de Sapo'
anticline (Fig. 15) to the south of the Mondoiiedo nappe.
Toward the foreland, an age of 336.5 ± 0.3 Ma was
obtained near La Espina thrust, at the limit with the external
zones to the east (Fig. 15). From these data, the development
of recumbent folding in the Mondoiiedo nappe can be
approximately placed somewhere in the interval between
360 and 335 Ma.
Two samples intended for dating the SI and S2 foliations
in the Mondoiiedo nappe gave ages of 300.0 ± 1.0
(4°Arf39Ar whole rock) and 298.2 ± 0.6 Ma (4°Arf39Ar
muscovite), respectively (Dallmeyer et aI., 1997). However,
Struchual evolution of the Mondoiiedo nappe and its footwall llllit showing the development of major extensional struchues dlUing thrusting. A black
dot at the rear edge of the Mondoiiedo thrust marks the suggested propagation of the fault into the basal ductile shear zone. The metamorphic isograds have
been included. (a) After recumbent folding and equilibration of Barrovian metamorphism. (b) Early stages of emplacement of the Mondoiiedo nappe and
coeval extension, related to the upper extensional shear zone. (c) As before, but extension begins to affect the footwall unit, associated with the ascent of
igneous rocks and partially molten gneisses. (d) and (e) Westward motion of the hot, partially molten rocks induces continued activity of the lower extensional
shear zone and detachment, and heating of its hanging wall unit. Note the ascent of the sillimanite isograd into the previous low-grade Xistral Quartzite, the
extension of this unit accomplished by domino-style boudinage, and the truncation of one of the boudins by the thrust fault, forming the Seixo Blanco slice. (f)
The Viveiro nonnal fault post-dates the last increments of thrusting.
340
330
320
310
300
290 Ma
TIME
Development of recumbent folds
Mondoiiedo and underlying
contractional shear zones
Upper extensional shear zone
Lower extensional shear system
Synkinematic granitoids
Late stages of thrusting. Tectonic slices
and brittle longitudinal extension
Viveiro fault
14. Timing of
(2000).
Fig.
contractional and extensional stmchues across the study area. Orronology is based on Dalbneyer et al.
synkinernatic granitoids gave U-Pb zircon and rnonazite
ages ranging between 310 and 330 Ma for the massifs of
Sarria, Viveiro and Penedo Gordo (Fernandez-Suarez et al.,
2000). This would suggest that both the SI and S2 foliations
are more than 10 rn.y. younger than the granitoids
synkinernatic with nappe emplacement. A more feasible
explanation is that, rather than dating fabric development,
the two 40ArP9Ar ages represent cooling ages related to
unroofing, and perhaps date the uplift of the nappe by
motion on the Viveiro fault.
The Sarria massif is a two-mica granite intruded in
the southern part of the Mondoiiedo nappe (Fig. 3) and
deformed in its basal shear zone (M:artinez Catalan,
1983, 1985). The intrusion also reached the upper
extensional shear zone, but is undeformed there (Figs.
3 and 4, section B -B') and assumed to post-date this
shear zone. Consequently, the age of crystallization of
the granite, 313 ± 2 Ma (Fernandez-Suarez et aI., 2000),
establishes an upper limit for the upper extensional shear
zone, whose motion pre-dated this age, and also a lower
limit for the final emplacement of the thrust sheet (Fig.
14), which continued moving after the granite intrusion.
The Penedo Gordo massif, dated at 31 T':; Ma (Fernan­
dez-Suarez et aI., 2000), also intruded into the upper
extensional shear zone, but was only deformed by the
Viveiro fault (Figs. 3 and 7), establishing a lower limit
for its motion and, as the Sarria massif, an upper age
limit for the upper extensional shear zone. The Viveiro
massif (Figs. 3, 7 and 8) was deformed in the lower
extensional shear zone, so that the 323�� Ma age of
crystallization obtained by Fernandez-Suarez et a1. (2000)
represents a lower limit for the late activity of this
structure.
Postkinematic granitoids have been dated between 285
and 295 Ma (U-Pb method, Fernandez-Suarez et aI.,
2000) and 275 and 285 Ma (4°Ar/39Ar method; Dall-
(1997) and Femandez-Smuez et
al.
meyer et aI., 1997). U-Pb data are more reliable for
crystallization ages, and provide an upper limit for thrust
tectonic activity: the A Tojiza massif (295 ± 2 Ma) and
the San Ciprian massif (286 ± 2 Ma) overprinted the
basal shear zone and thrust fault of the Mondoiiedo
nappe (Fig. 8, sections 1-1' and IT-IT'). These same
massifs also yielded 40Ar/39Ar muscovite ages of 284 and
274 Ma, respectively, suggesting a time lapse of 10 my
between crystallization and cooling to the closure
temperature of argon in muscovite.
Taking together these data and the mutual relation­
ships between the different structures described in the
previous sections, a picture of the structural evolution
can be traced.
After a first episode of recumbent folding, crustal
thickening and burial, loosely constrained between 360
and 335 Ma, a Barrovian metamorphic zoning was
established and equilibrated (Fig. 13a and Fig. 6, paths
A, B and C, prograde part of grey arrows, mostly
dashed). The Xistral Quartzite occupied, at that time, a
relatively shallow position.
In the next 30 million years, the contractional ductile
shear zones developed. The more internal parts of the
thrust sheet initiated their exhumation along the basal
shear zone of the Mondoiiedo nappe (Fig. 13a and b)
from a depth of 38-45 km (Arenas and Martinez
Catalan, 2003). While thrust motion induced crustal
thickening, the thrust sheet became thinned by the upper
extensional shear zone, and the lower extensional
detachment developed in the footwall unit (Fig. 13c).
An upper age limit for the extensional activity in the
nappe is provided by the Sarria and Penedo Gordo
massifs (313 ± 2 and 31 T':; Ma, respectively; Fernan­
dez-Suarez et aI., 2000), which were not affected by the
upper extensional shear zone.
Heat accumulation due to crustal thickening and,
Covered by seismic profile
,
--1.
ESCIN-3.3 .
1-------,
I
Internal zones
Central Iberian
Zone
Covered
by seismic profi le
1----, ESCIN-1 (ESCICANTABRICA-1) --l
.,------+--, External zone +-----i
West Asturian-/eonese Zone
I
Cantabrian Zone
25
50
km
C
A NT
A BRI
A N ZONE
DE A
T CH ME N T
IIIIII
�
IZ::l
D
Fig.
ALLOCHTHONOUS TERRA NES
ARMORICAN QUA RTZITE
"OLLO DE SAPO" FM.
PRECAMBRIAN SLATES:
VILALBA AND NARCEA FM.
15.
IIIIIi
-[Z;j
�
-
BIOTITIC POSTKINEMATIC GRANITOIDS
TWO-MICA GRANITOIDS
BIOTITIC SYNKINEMATIC GRANITOIDS
MANTLE
The struchue of the Variscan wedge, as suggested at the end of convergence, based on smface geology and geophysics.
probably, to advention of mantle-derived rocks, may have
triggered the development of the lower extensional shear
zone, which juxtaposed deep-seated hot rocks against the
overlying and relatively cold Xistral Quartzite. The thick
and competent quartzitic layer might have acted as a screen,
channelling the viscous flow of ascending buoyant material.
Partially molten crustal and subordinate mantle rocks trying
to open their way upward (Fig. 6, path G) would have been
obliged to flow near horizontally, giving rise to the lower
extensional detachment (Fig. 13c-e). Heating was very
strong in the footwall to the Mondoiiedo nappe (Fig. 6, paths
E and F), and synkinematic igneous rocks intruded in the
footwall to the lower detachment.
The heat transmitted from below annealed the mylonitic
quartzites of the hanging wall to the detachment, and also
those of the two contractional shear zones in the Xistral
tectonic window, that had previously developed low-grade
mylonites (Fig. 1 1). Consequently, we assume that the lower
detachment post-dated part of the ductile shearing in the
Mondoiiedo and lower contractional shear zones. However,
it seems that heating was accompanied with further ductile
deformation, so that all of the shear zones in the footwall to
the Mondofiedo thrust fault were heated while being still
active. Furthermore, as long as the Mondoiiedo thrust and
associated tectonic slices cut the annealed mylonites, we
infer that thrust movement continued after the activity along
the lower detachment had ceased.
Once thinned, the Mondoiiedo nappe moved as a
relatively cold thrust sheet, less than 20 km thick, over­
thrusting its present footwall unit, which registered a
moderate heating and pressurization until the P-T con­
ditions at the bottom of the hanging wall (Fig. 6, path C)
were equal to those at the top of the footwall (Fig. 6, path
D). The thrust developed as a discrete fault close to the
ductile-brittle transition, propagating into the previous
ductile shear zone and preserving most of it in the hanging
wall (Fig. 13c-e), but also a portion in the footwall (Fig. 7).
The Xistral Quartzite underwent E-W and N-S
extension coeval with the late stages of nappe emplacement.
Ductile extension in the E-W direction included domino­
style rotation of individual blocks (Fig. 13d). N-S extension
was locally ductile but most generally brittle (Fig. 10). In
both cases, asperities created by the individual blocks,
equivalent to megaboudins, were cut by the late thrust fault,
giving rise to lens-shaped tectonic slices (Figs. 10 and 13e).
However, this was not always the case, because the faults
bounding horsts and grabens created by the N-S extension
acted as lateral ramps, giving rise to the transverse E-W­
trending open folds in the thrust sheet (Figs. 3, 7, 9 and 10).
Once thrust motion had ceased, the Vi veiro fault
developed, crosscutting the Mondofiedo thrust sheet and its
autochthon (Fig. 13f).
7. hnplications for the dynamics of the Variscan
orogenic wedge
The Mondofiedo nappe occupies the western part of
the West Asturian-Ieonese Zone, which is one of the
internal zones of the Variscan belt in the NW Iberian
Massif (Julivert et aI., 1972). To the east, the
Cantabrian Zone is a foreland thrust belt representing
an external zone of the massif. Fig. 15 sketches the final
stage of the orogenic evolution in the Mondofiedo nappe
and surrounding areas, as deduced from deep seismic
reflection profiles (Perez-Estaun et aI., 1994; Ayarza
et aI., 1998), and structural information from surface
geology after Marcos (1973), Bastida et al. (1982,
1986), Perez-Estaun et al. (1988, 199 1), Martinez
CataJan et al. ( 1990), Gutierrez-Alonso (1996), and
this work.
A deep reflection seismic profile acquired on land in the
Cantabrian Zone (ESCIN-l ) shows a wedge geometry and a
relatively shallow sole thrust, confirnilng the thin-skinned
style of deformation in this zone (perez-Estaun et aI., 1994).
In another deep seismic profile (ESCIN-3.3) acquired
offshore north of the study area, a crustal-scale thrust is
suggested by a series of reflections below the Mondofiedo
nappe, which continue west, down-dip into the lower crust
and the Moho discontinuity (Ayarza et aI., 1998). These
structures, called, respectively, the Cantabrian Zone detach­
ment and the sole thrust of the West Asturian-Ieonese Zone,
have been incorporated into Fig. 15. Furthermore, the
ESCIN-3.3 profile shows two or three bands with high
reflectivity in the lower half of the crustal section underlying
the Mondonedo nappe. Ayarza et al. (1998) interpreted them
as lower crust-mantle imbrications, because refraction
seismics and a magnetic anomaly suggest the existence of
mafic and ultramafic rocks relatively close to the surface
(Aller et aI., 1994; Cordoba et aI., 1987; Ayarza et aI.,
1998).
All the geological and geophysical data can be readily
integrated with the structural evidence to draw an image
of the orogenic belt as a wedge, and also to gain insights
into its evolution. The fact that the structural style of the
Cantabrian Zone is thin-skinned suggests that its base­
ment must have been underthrust beneath the West
Asturian-leonese Zone. For the 90-km-long ESCIN-l
profile, the accumulated displacement of the thrusts faults
has been estimated as 150 km (P6rez-Estaun et aI., 1994),
implying that another 60 km should have been moved to
the west. The Cantabrian Zone continues at least 80 km
eastward of the end of the seismic profile, so that the
total length of the underthrust basement should be more.
Thin-skinned tectonics were active in the Cantabrian
Zone until Kasimovian times (perez-Estaun et aI., 1988),
i.e. until 300 m.y. ago, and this is the age suggested for
the Viveiro fault by 4oArl39Ar cooling ages in the
Mondonedo nappe (Dallrneyer et aI., 1997). Furthermore,
the lowermost band with high reflectivity in profile
ESCIN-3.3, situated at a depth of 34-41 km, can be
followed to approximately the vertical of the Viveiro
fault.
According to these data, a wedge shape is quite
reasonable for this area of the Variscan belt. The innermost
part of the original basement of the Cantabrian Zone would
have acted as the lower boundary of the wedge during the
late stages of convergence, and its lll1derthrusting would
have been active until around 300 m.y. ago. In spite of being
subjected to continuous (or episodic) shortening (Fig. 14),
two extensional shear zones developed inside the wedge,
probably to compensate gravitational gradients (Hodges
et aI., 1996) created by its internal dynanilcs and thermal
evolution. The fact that the lower extensional shear zone
developed beneath the Mondonedo nappe during its
emplacement confirms that its basal thrust was a contra­
tional fault inside the wedge, and not its basal boundary.
Underplating probably played a role in the extensional
activity, as frontal accretion to the east occurred in a thin­
skinned mode; that is, involved only the sedimentary prism.
Westward underthrusting of part of the the external
basement seems to have occurred partially beneath the
lower continental crust and the subcontinental mantle of the
internal zones (Fig. 15). Buoyancy due to this density
inversion may be responsible for increasing the vertical
stress and creating topographic relief. The same effect
would have been induced by partial melting deep in the
crust. In addition, prolonged heating, mostly due to orogenic
thickening but possibly with a magmatic contribution from
the mantle (GaJan et aI., 1996), would have lowered the
viscosity of the middle and lower crust, preparing it to flow
easily in response to the internal stress field. If vertical
stresses were dominant in the deep parts of the wedge, the
viscous flow would have extended it, mostly transversally,
i.e. in the E-W direction (although some longitudinal
extension also took place), inducing a decrease in the wedge
taper.
The lower extensional shear zone may represent a
transition between a low-viscosity, flowing crustal layer
and a stronger crust above. In fact, this broad structure,
whose lower limit does not outcrop, might have accom­
modated much of the internal strain needed to extend and
sharpen the wedge. Conversely, the upper extensional shear
zone probably developed in the relatively strong part of the
wedge to reduce topographic relief and/or to accommodate
the E-W extension lll1dergone by deeper parts. Extension in
both shear zones took place while convergence continued,
as demonstrated by crosscutting relationships of the late
Mondonedo thrust (Figs. 13 and 14). The Viveiro fault,
post-dating nappe emplacement, could reflect the latest
stages of convergence in the Cantabrian Zone.
8. Conclusions
2.5D interpretation of the eastern Galicia magnetic anomaly (north­
western Spain):
Vertical shortening and extension occurred in both the
hanging wall and the footwall to the Mondonedo thrust fault
synkinematically with thrusting, as shown by crosscutting
structural relationships and overprinting metamorphic
criteria from two extensional shear zones. Extension was
mostly transversal to the orogenic trend, but subordinate
longitudinal extension induced normal faulting in the
footwall unit to the Mondoiiedo nappe, and some of these
faults were used as lateral ramps in the latest stages of
thrusting.
Using published geological and geophysical data, the
complex 3D structural evolution deduced for the Mondo­
iiedo nappe and its autochthon is viewed in a wider regional
context, as forming part of an orogenic wedge active during
most of the Carboniferous in the NW Iberian Massif. In its
late stages, gliding of the wedge could have taken place over
the presently missing part of the original basement of the
foreland thrust belt, which should have been carried beneath
the internal zones while its sedimentary cover was being
peeled off and imbricated in the front of the wedge.
Heterogeneity characterize the internal deformation of
the wedge, with the strain partitioned into structures of
different significance. In the case examined in this paper, a
major thrust led the local kinematics, overprinting all the
extensional structures developed, except the last. This
seems to reflect the regional stress field, clearly dominated
by plate convergence. However, heat accumulation result­
ing from crustal thickening and magmatic underplating
weakened the deep parts of the wedge, giving rise to a
viscous flow that accommodated its extension, induced by
gravitational instabilities. The lower extensional shear zone
probably represents the transition between a low-viscosity
flowing mass and an overlying, more viscous structural
level, with the strain being concentrated in an important
lithological boundary, the base of the competent Xistral
Quartzite. The upper extensional shear zone, developed in
relatively higher and stronger parts of the wedge, would
result from E-W stretching, gravitational collapse or both.
geodynamical implications. Tectonophysics 237,
201-213.
AranglUen, A, Tubia, 1.M., 1992. StruCtlUal evidence for the relationship
between thrusts, extensional faults and granite intrusions in the
Variscan belt of Galicia (Spain). lOlUnal of Structural Geology 14,
1229-1237.
Arenas, R, Martinez Cataloin, lR, 2003. Low-P metamorphism following
a Barrovian-type evolution. Complex tectonic controls for a common
transition, as deduced in the Mondoiiedo thrust sheet (NW Iberian
Massif). Tectonophysics in press.
Ayarza, P., Martinez Cataloin, lR, Gallart, 1., Daiiobeitia, 1.1., Pulgar, 1.A,
1998. Esmdio Sismico de la Corteza Iberica Norte 3.3: a seismic image
of the Variscan crust in the hinterland of the NW Iberian Massif.
Tectonics 17, 171-186.
Bastida, F., Pulgar, 1.A, 1978. La estruchua del Manto de Mondoiiedo
entre BlUela y Tapia de Casariego (Costa Cantabrica, NW de Espaiia).
Trabajos de Geologia, Universidad de Oviedo 10, 75 -124.
Bastida, F., Marcos, A, Marquinez, 1., Martinez Cataloin, 1.R, Perez­
EstaUn, A, Pulgar, 1.A, 1982. Mapa Geologico Nacional, mstituto
Geologico y Minero de Espaiia, sheet 1, La Coruiia, scale 1:200,000.
Bastida, F., Martinez Cataloin, 1.R,
Pulgar,
l.A.,
1986. StruCtlUal,
metamorphic and magmatic history of the Mondoiiedo nappe
(Hercynian belt, NW Spain). lournal of StructlUal Geology 8, 415-430.
Beaumont, C., lamieson, RA, Nguyen, MH., Lee, B., 2001. Himalayan
tectonics explained by extrusion of a low-viscosity crustal channel
coupled to focused sutface denudation. Nahue 414, 738-742.
Behrmann, 1.H., Platt, 1.P., 1982. Sense of nappe emplacement from quartz
c-axis fabrics; an example from the Betic Cordilleras (Spain). Earth and
Planetary Science Letters 59, 208-215.
Block, L., Royden, L.H., 1990. Core complex geometries and regional scale
flow in the lower crust. Tectonics 9, 557-567.
Bouchez, 1.L., Pecher, A, 1981. The Himalayan Main Central Thrust Pile
and its quartz-rich tectonites in Central Nepal. Tectonophysics 78,
23-50.
BlUchfiel, B.C., Royden, L.H., 1985. North-south extension v.rithin the
convergent Himalayan region. Geology 13, 679-682.
BlUg, 1.P., Bnmel, M., Gapais, D., Chen, G.M., Liou, G.H., 1984.
Defonnation of leucogranites of the crystalline Main Central Sheet in
southern Tibet (Grina). lomnal of Struchual Geology 6, 535-542.
BlUg, 1.P., Van Den Driesche, 1., Bnm, 1.P., 1994. Syn- to post-thickenillg
extension: mode and consequences. Compte Rendue de la Academie
des Sciences, Paris 319, 1019- 1032.
Butler, RWH., 1982. Hanging wall strain: a fllllction of duplex shape and
footwall topography. Tectonophysics 88, 235-246.
Capdevila, R, 1969. Le metamorphisme regional progressif et les granites
dans le segment hercynien de Galice nord oriental (NW de I 'Espagne).
Ph.D. Thesis, Universite de Montpellier.
Gark, M.K., Royden, L.H., 2000. Topographic ooze: building the eastern
margin of Tibet by lower crustal flow. Geology 28, 703-706.
Acknowledgements
Cordoba, D., Banda, E., Ansorge, 1., 1987. The Hercynian crust in
northwestern Spain: a seismic slUvey. Tectonophysics 132, 321-333.
Dennis Brown and Chris Wibberley are sincerely
acknowledged for their critical reviews of the early version
of the manuscript. Their clever comments and suggestions
and the exhaustive correction of English grammar helped to
improve the quality of the paper.
Costa, S., Rey, P., 1995. Lower crustal rejuvenation and growth dlUing
post-thickening collapse: msights from a crustal cross section through a
Variscan metamorphic core complex. Geology 23, 905 -908.
Coward, M.P., 1982. SlUge zones in the Moine thrust of NW Scotland.
lournal of Struchual Geology 4, 247-256.
Dahlen, F.A, Suppe, 1., Davis, D., 1984. Mechanics offold-and-thrust belts
and accretionary wedges:
cohesive Coulomb theory. lOlUnal of
Geophysical Ressearch 89, 10087-10101.
References
Dahlstrom, C.D.A, 1970. StrUctlUal geology in the eastern margin of the
Canadian Rocky MOlllltainS. Bulletin of Canadian Petroleum Geology
18. 332-406.
Aller, 1., Bastida, F., 1993. Anatomy of the Mondoiiedo Nappe basal shear
zone (NW Spain). 10lunal of Struchual Geology 15, 1405-1419.
Aller, 1., Zeyen, H.l., Perez-EstaUn, A, Pulgar, 1.A, Pares, 1.M., 1994. A
Dalbneyer, RD., Martinez Catal:lli., 1.R, Arenas, R, Gi1 Ibarguchi, 1.1,
Gutierrez Alonso, G., Farias, P., Aller, 1., Bastida, F.,
1997.
Diachronous Variscan tectonothermal activity in the NW Iberian
Massif: evidence from 4°ArP9Ar dating of regional fabrics. Tectono­
de la Peninsula Iberica y Baleares. Instituto Geol6gico y Minero de
physics
Espaiia, Madrid, scale
277, 307-337.
Davies, H.L., Warren, RG.,
1988.
Origin of eclogite-bearing, domed,
layered metamorphic complexes ("core complexes") in the D'Entre­
casteaux Islands, Papua New Guinea. Tectonics
1153-1172.
Dewey, l.P., 1988.
1123-1139.
1983. Mechanics offold-and-thrust belts
lournal of Geophysical Research 88,
Extensional collapse of orogens. Tectonics
Dietrich, D., Casey, M.,
1989.
7,
nappes. In: Coward, M.P., Dietrich, D., Park, RG. (Eds.), Alpine
45,
pp.
47 -63.
Eliiott, D., lolmson, M.RW.,
1980. Structlual evolution in the northern part
N\V Scotland. Transactions of the Royal
of EdinblUgh, Earth Sciences 71, 69-96.
of the Moine thrust belt,
Society
England, P.C.,
Some numerical investigations of large scale
1983.
continental defonnation. In: Hsu, K.J.,
Processes, Academic Press, London, pp.
(Bd.), MOlmtain Building
129-139.
Escuder Viruete, l., Arenas, R, Martinez Cataloin, l.R,
1994. Tectonother­
mal evolution associated with Variscan crustal extension in the Tonnes
Gneiss Dome
(NW" Salamanca, Iberian Massif,
Spain) . Tectonophysics
238, 117-138.
Escuder Vimete, l., Indares, A, Arenas, R,
1997. P-T path determinations
N\V Iberian Massif, Spain. lOlUnal of
15, 645-663.
in the Tonnes Gneissic Dome,
Metamorphic Geology
Fernoindez-Suarez, l., Dunning, G.R, lellller, G.A, Gutierrez-Alonso, G.,
2000.
Variscan collisional magmatism and deformation in
N\V Iberia:
constraints from U-Pb geochronology of granitoids. Journal of the
Geological Society, London
Fossen,
H, 1992.
157, 565-576.
The role of extensional tectonics in the Caledonian of
south Norway. lOlUnal of Struchual Geology
14, 1033-1046.
2000. Extensional tectonics in the Caledonides: synorogenic or
Fossen, H.,
postorogenic? Tectonics
Galoin, G.,
1987.
19, 213-224.
Las rocas graniticas del Macizo de Vivero en el sector
Norte (Lugo, NO de Espaiia) . Corpus Geologicum Gallaeciae,
vol.
2a Serie,
3.
Galoin, G., Pin, C., Duthou, l.L.,
1996. Sr-Nd isotopic record of multi-stage
interactions between mantle-derived magmas and crustal components
in a collision context-the ultramafic-granitoid association from
Vivero (Hercynian belt,
Gutierrez-Alonso, G.,
N\V Spain) . Chemical Geology 131, 67-91.
1996. Strain partitioning in the footwall of the
Somiedo Nappe: struchual evolution of the Narcea Tectonic Window,
N\V
Spain. lOlUnal of StruCtlUal Geology
Harris, L.D.,
18, 1217-1229.
1970. Details of thin-skinned tectonics, in parts of Valley and
Ridge and Cumberland Plateau provinces of the southern Appalachians.
ill: Fisher, G.W., Pettijolm, F.J., Read, l.C., Weaver, KN. (Bds.),
Studies of Appalachian Geology: Central and Southern, illterscience,
New York, pp.
161-173.
Hatcher, RD. Jr, Hooper, Rl.,
1992. Evolution of crystalline thrust sheets
in the internal parts ofmOlmtain chains. In: McClay, KR, (Bd.), Thrust
Tectonics, Chapman
& Hall,
Hatcher, RD. Jr, Williams, RT.,
sheets. Part
1.
London, pp.
217-233.
1986. Mechanical model for single thrust
Taxonomy of crystalline thrust sheets and their
relationships to the mechanical behaviolU of orogenic belts. Geological
Society of America Bulletin
Hodges, K.V., Walker, l.D.,
97, 975-985.
1992.
Extension in the Cretaceous Sevier
orogen, North American Cordillera. Geological Society of America
Bulletin
104, 560-569.
1979.
1996.
Tectonic evolution of the
central Annapurna Range, Nepalese Himalayas. Tectonics
15,
1264-1291.
theoretical controls and obsetved phenomena. lomnal of Struchual
Geology
Marcos, A,
1, 283-297.
1973. Las Series
del Paleozoico Inferior y la estruchua
Geologia, Universidad de Oviedo
(N\V
6, 1-113.
1998.
Detachments in high-pressure mOlmtain belts, Tethyan examples. Earth
lulivert, M., Fontbote, l.M., Ribeiro, A, Conde, L.,
1972. Mapa Tect6nico
1996.
Struchual
and metamorphic evidence of local extension along the Vivero fault
coeval with bulk crustal shortening in the Variscan chain
lomnal of Struchual Geology
Martinez Cataloin, l.R,
(NW" Spain).
18, 61-73.
L'apparition du chevauchement basal de la
1980.
nappe de Mondoiiedo dans le dome de Lugo (Galice, Espagne). Copmte
Rendue de la Academie des Sciences, Paris
Martinez Cataloin, l.R,
1983.
290, 179-182.
Deformaci6n heterogenea en los macizos
graniticos de Sarria y Santa Eulalia de Pena (provincia de Lugo). Stvdia
Geologica Salmanticensia
Martinez Cataloin, lR,
18, 39-63.
1985. Estratigrafia y
estruchua del domo de Lugo
(Sector Oeste de la zona Ashuoccidental-Ieonesa). Corpus Geologicum
Gallaeciae,
2a
Serie, vol.
2.
Martinez Cataloin, lR, Perez Estalin, A, Bastida, P., Pulgar, lA., Marcos,
A,
1990. West Ashuian-Leonese Zone.
StructlUe. ill: Dalhneyer, RD.,
Martinez Garcia, E. (Bds.), Pre-Mesozoic Geology of Iberia, Springer­
Verlag, Berlin, pp.
Matte, P.,
1968.
103-114.
La structlUe de la virgation hercynielllle de Galice
(Espagne). Revue de Geologie Alpine 44,
Milnes, AG., Wellllberg, O.P., Skar,
0.,
1 - 128.
Koestler, AG.,
1997. Contraction,
extension and timing in the South NOIWegian Caledonides: the
Sognefjord transect. ill: BlUg, l.P., Ford, M. (Eds.), Orogeny Through
Time. Geological Society Special Publication
121, pp. 123-148.
1961. Evolution of metamorphic belts. lOlUnal of Petrology
2, 277-311.
Molnar, P., Lyon-Caen, H., 1988. Some simple physical aspects of the
Miyashiro, A,
support, structlUe, and evolution of mOlmtain belts. Geological Society
of America Special Paper
Morley, c.K.,
1988.
218, 179-207.
Out-of-sequence thrusts. Tectonics
Mueller, K., Talling, P.,
1997.
7, 539-561.
Geomorphic evidence for tear faults
accommodating lateral propagation of an active fault-bend fold,
Wheeler Ridge, California. lOlUnal of Struchual Geology
Perez-Estalin, A,
19, 397 -411.
1978. Estratigrafia y estruchua de la rama Slu de la Zona
Ashuoccidental-Leonesa. Memorias del illstituto Geol6gico y Minero
de Espaiia
92, 1-151.
Perez-Estalin, A , Bastida, P., Alonso, l.L., Marquinez, l . , Aller, l.,
Alvarez-Marr6n, l., Marcos, A, Pulgar, lA.,
1988.
A thin-skinned
tectonics model for an arcuate fold and thrust belt: the Cantabrian Zone
(Variscan Ibero-Annorican Arc). Tectonics
7, 517-537.
Perez-Estalin, A, Martlnez Cataloin, lR, Bastida, F.,
1991.
Crustal
thickenning and deformation sequence in the footwall to the suhue of
the Variscan Belt of northwest Spain. Tectonophysics
191, 243-253.
Perez-Estalin, A, Pulgar, lA., Banda, E., Alvarez-Marr6n, l., ESCI-N
Research Group,
1994. Crustal struchue of the external Variscides in
N\V Spain from deep seismic reflection profiling. Tectonophysics 232,
91-118.
Pitra, P . , BlUg, l.P., Schulmann, K., Ledru, P., 1994. Late orogenic
extension in the Bohemian Massif: petrostruchual evidence in the
Platt, l.P.,
1984.
1986.
15-30.
Secondary cleavages in ductile shear zones. lomnal of
Struchual Geology
6, 439-442.
Dynamics of orogenic wedges and the uplift of high­
presslUe metamorphic rocks. Geological Society of America Bulletin
97, 1037-1053.
1993. Exhumation of high-presslUe rocks:
and processes. Terra Nova 5, 119- 133.
Platt, l.P.,
160, 31-47.
de Espaiia). Trabajos de
Martinez, F.J., Carreras, l . , Arboleya, M.L., Dietsch, C . ,
Platt, l.P.,
lolivet, L., Goffe, B., Bousquet, R, Oberhiinsli, R, Michard, A,
2, 355-371.
Fabric development in shear zones:
lllinsko region. Geodinamica Acta 7,
Hodges, K.Y., Panish, RR, Searle, M.P.,
and Planetary Science Letters
Lister, G.S., Williams, P.P.,
herciniana del occidente de Ashuias
A new tectonic model for the Helvetic
Tectonics. Geological Society Special Publication
simulation of fabric development
during plastic defonnation and its application to quartzite: the influence
of defonnation history. lomnal of Struchual Geology
7, 1-21.
Davis, D., Suppe, l., Dahlen, FA.,
and accretionary wedges.
1:1,000,000.
1980. The
Lister, G.S., Hobbs, B.E.,
a review of concepts
1989.
Platt, l.P., Vissers, R.LM.,
Extensional collapse of thickened
continental lithosphere: a working hypothesis for the Alboran Sea and
Gibraltar arc. Geology
17, 540-543.
1998.
Evolution of a kyanite-bearing belt within a HT -LP oro gen: the case of
NW Variscan Iberia. lomnal of Metamorphic Geology
1989. Horizontal struchues in
SM., Casey, M.,
1986.
16, 379-394.
granulite terrains: a record of
mOlmtain building or mountain collapse? Geology
Schmid,
17, 449-452.
Complete fabric analysis of some
Paterson Volume.
Geophysical
36, American Geophysical Union, pp. 263-286.
Royden, L.H., BlUchfield, B.C., 2001. Large-scale
Monograph
Shen, F.,
crustal
defonnation of the Tibetan Plateau. lomnal of Geophysical Research
106, 6793-6816.
Thompson, A.B., England, P.c.,
of regional metamorphism,
ll.
1984.
PresslUe-temperatlUe-time paths
Their inference and interpretation using
mineral assemblages in metamorphic rocks. lOlUnal of Petrology
25,
929-955.
Van Den Driessche, l., Bnm, l.P.,
0.,
1991- 1992.
Tectonic evolution of the
5, 85-99.
Burg, l.P., Teyssier,
C.,
1999.
Exhumation of
migmatites in two collapsed orogens: Canadian Cordillera and French
Variscides.
ill:
Rillg, U., Brandon, M.T., Lister, G.S., Willett, S.D.
(Eds.), Exhumation Processes: Normal Faulting, Ductile Flow and
154, pp. 181 -204.
1994. Criteria for identifying struchues related
Erosion. Geological Society Special Publication
"Wheeler, l., Butler, R.W.H.,
commonly observed quartz c-axis patterns. Mineral and rock defor­
mation. Laboratory Studies-The
dome. Geodinamica Acta
Vanderhaeghe,
Reche, l., Martlnez, F.l., Arboleya, M.L., Dietsch, C., Briggs, W.D.,
Sandiford, M.,
Montagne Noire (french Massif Central): a model of extensional gneiss
to true crustal extension in orogens. lomnal of Struchual Geology
16,
1023-1027.
Wibberley, C.A.J.,
1997.
Three-dimensional geometry, strain rates and
basement deformation mechanisms of thrust-bend folding. lomnal of
Struchual Geology
Wilson, C.J.L.,
1973.
19, 535-550.
The prograde microfabric in a defonned quartzite
sequence, MOllllt Isa, Australia. Tectonophysics
Yin, A,
Kelty, T.K.,
1991.
19, 39-81.
Development of normal faults during
emplacement of a thrust sheet: an example from the Lewis allochthon,
Glacier National Park, Montana (USA). lOlUnal of StructlUal Geology
13, 37-47.